1. Field of the Invention
The present invention relates to the field of optical instrumentation methods and devices to be employed in industrial instrumentation systems and the like, and in particular, to method and device for detecting and transmitting information only by optical means.
2. Description of the Prior Art
An industrial instrumentation system is generally configured by connecting numbers of sensors to process controllers. In such industrial instrumentation systems, the number of cables to be employed for transmitting data detected by individual sensors inevitably becomes very large as the system scale becomes enormous. The problems arisen from the great number of the cables has become a serious technical theme to be solved.
For this reason, it has been desired to establish a sensor network system wherein cables from a plurality of sensors are connected to a single cable and information detected by each sensor is tramsmitted through a single cable. For overall system safety, it has also been desired that the transmission system including sensors should be explosion proof. Furthermore, enhancement of system reliability has been desired.
From the requirements mentioned above, a sensor system employing only optical means is drawing attention. As such an optical sensor system employing optical means, there has been known an optical sensor system wherein a plurality of sensors are connected in parallel and information detected by each sensor is transmitted in time shared multiplex manner. FIG. 1 shows the basic configuration of such conventional system, in which S1, S2, and S3 represent optical switches which perform connection and disconnection of the optical path corresponding, for example, to the ON/OFF of the valve (not shown), and .tau.2 and .tau.3 represent delay optical fibers. When a light pulse is sent from the process controller (not shown) through l1, the information representing ON/OFF of the optical switches S1-S3 multiplexed in a time-shared manner by means of the delay optical fiber .tau.2, .tau.3 and then collected to transmit through a single line l2.
However, due to the fact that the delay time of an average delay optical fiber is about 1 msec/200 m, and that a long delay fiber cannot be adopted because of economical reason, the time slot to be assigned to each sensor becomes short, and as a result, such systems have not been adaptable to a high precision analog sensor.
On the other hand, there has been a conventional system of another type wherein digital sensors are employed as high precision sensors. In this case, however, since there is no means to multiplex each digit information to transmit it to the process controller, such a system must transmit each digit information through respective transmission line, presenting a problem of practicability.